Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.
Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above-stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The "orange peel" surface problem has caused thermosetting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems that may be occasioned by the evaporation of the solvent system. Solvent-based coating compositions also suffer from the disadvantage of relatively poor percent utilization, i.e., in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent-based coatings can be wasted since that portion which does not contact the article or substrate being coated obviously cannot be reclaimed.
In addition to exhibiting good gloss, impact strength and resistance to solvents and chemicals, coatings derived from thermosetting coating compositions must possess good to excellent flexibility. For example, good flexibility is essential for powder coating compositions used to coat sheet (coil) steel which is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles wherein the sheet metal is flexed or bent at various angles.
It is well known that the weatherability of coatings based on acrylic resins is superior to the weatherability of coatings based on widely used polyester resins. However, performance properties of acrylic coatings, such as impact, flexibility, and adhesion, are poor compared to the excellent performance properties of polyester coatings. Past attempts to provide coatings with both good weatherability and flexibility by blending acrylics and polyesters have resulted in only a marginal increase in performance.
The present invention, as described below, teaches that coatings formulated with blends of glycidyl-containing acrylic resins and novel carboxyl-containing semicrystalline polyesters have both the excellent weatherability of acrylic coatings and the performance properties of polyester coatings.
Japanese Kokai 57-2054458 (1981) discloses powder coatings comprising carboxyl-containing polyester, glycidyl-containing acrylic polymer, and carboxyl-containing vinyl polymer that are claimed to have excellent weather resistance and physical properties.(see also Japanese Kokai 60-120764 (1983) 57-2054458.)
U.S. Pat. No. 4,499,239 discloses a resin composition for powder coating consisting of a polyester resin and a glycidyl-containing acrylic polymer.
U.S. Pat. No. 4,818,791 discloses powder paint comprising (A) a polyester resin containing both hydroxyl and carboxyl groups (B) an vinyl polymer containing both glycidyl and hydroxyl groups and (C) a blocked isocyanate.
U.S. Pat. No. 4,824,909 describes a powder coating composition comprising (A) a polyester resin having both acid and hydroxyl functionality, (B) an acrylic with a hydroxyl and/or a glycidyl group, and (C) a blocked isocyanate.